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14.3 Translation
tRNA
• A molecule heavily involved in translation is tRNA (transfer
RNA).
• These molecules are used to carry amino acids, so that
they are in close vicinity to each other.
• The open-end of a tRNA
molecule is the ‘amino acid
attachment site.’
• The opposite end is the
‘anticodon loop.’
• The anticodon loop consists
of 3 unpaired bases.
• These bases vary, giving
tRNA specificity to amino
acids.
The Sequence of Events....
Transcription
Splicing
Amino Acid Activation
Translation
Amino Acid Activation
• You now know that tRNAs carry specific amino acids.
• However, you need to know that a tRNA cannot pick up an
amino acid unless the amino acid is activated.
• The amino acid attachment site of a tRNA will bind to a
specific amino acid, if energy is supplied.
So... What’s happened so far?
We’ve created a strand of mRNA in
transcription...
We’ve spliced it and taken out the 98%
of ‘junk DNA’...
We’ve activated a load of amino acids
and stuck them to tRNA molecules...
RNA polymerase
DNA helicase
dna
When RNA polymerase
reaches a particular
sequence of bases on the
DNA (stop triplet!), it
detaches, and the
production of pre-mRNA is
complete.
Template Strand of DNA (a gene)
The coding sections are
called EXONS.
The NON-coding
sections are called
INTRONS.
Transcription (production of
pre-mRNA
Once the useful exons are
removed from pre-mRNA,
they are spliced together
to form a final mRNA
strand.
Translation is now ready to
happen...
In the cytoplasm...
Peptide Bond
Forms
His
Peptidyl Transferase
Met
His
Arg
G U A
U
A C
G U G
U
A U G C
A
C
A G G
C
C C
A U U A A
STOP CODON!!
RELEASE FACTOR
In Words...
1. Fresh out of transcription, the mRNA strand associates with a
ribosome (in the cytoplasm).
2. The 1st 2 codons are now in the ribosome and attract their
complementary tRNA molecules (which are carrying their specific
amino acids).
3. A peptide bond is formed between the two amino acids (catalysed
by peptidyl transferase).
4. The ribosome moves forwards by one codon (3 bases).
5. The first tRNA leaves the ribosome, while a new tRNA enters it,
carrying it’s amino acid towards the newest codon.
6. Another peptide bond is formed between the 2nd and new tRNAs.
7. The ribosome continues moving along the mRNA until it reaches a
stop codon. There, it will detach due to the presence of a release
factor.
8. The polypeptide chain is then released, folding into the correct
shape.
Multiple Ribosomes Working on a Single mRNA Strand
• In reality, many ribosomes work simultaneously along a
strand of mRNA.
• This massively speeds up protein synthesis and increases the
protein yield from a single mRNA strand.
• Several ribosomes working their way along a single mRNA
strand are called a polysome.